This invention pertains to a strapping machine. More particularly, this invention pertains to an improved tension, seal and feed arrangement for a strapping machine for plastic strapping material.
Strapping machines are well-known in the art. There are two principle types of strapping machines, manual and automatic, table-top or free-standing machines. Strapping machines are typically designed for use with either plastic or metal (steel) strapping. These machines position, tension and seal strap around a load to bundle or secure the load.
A typical automatic strapping machine includes a frame-like support for the overall machine, a working area to, for example, support a load, a feed head to feed strap around the load and to retract strap prior to tensioning, a chute through which the strap is fed around the load, a strapping head to secure the strap to itself and one or more dispensers for dispensing the strap material to the strapping head.
In a plastic strapping machine, the strapping head serves a number of functions. First, it clamps or grips a free end of the strap as it returns to the strapping head, it clamps or grips a trailing end of the strap following retraction and during tensioning of the strap and it grips the strap again on the opposite or inside of the strap loop, as it severs the sealed strap from the feed or dispenser side and seals the overlapping courses of strapping together.
The feed head typically includes one or more pairs of rotating wheels to feed strap into the strapping head, around the strap chute and back to the strapping head. The feed head also includes a retraction arrangement to retract the slack strap and to “pull” the strap from the strap chute onto the load or package to be strapped. The feed head also includes a tension arrangement to pull tension in the strap to secure the load prior to forming a seal in the strap loop. Known strapping machine feed heads include two pairs of rotating wheels to carry out the feed, retraction and tension functions. Others include a single pair of wheels to carryout these functions. In each, typically, one of the wheels of each pair is a driven wheel and the other wheel is an idler wheel (to provide a surface against which the driven wheel rotates). Still other arrangements use a windlass or winder around which the strap is wound to pull tension in the strap. The winder can be located in the strapping head, in the feed head, or between the heads.
While these arrangements function well to tension the strap, they do so by pulling the strap from one end in an effort to tension the strap along the entire length of the strap as it is wrapped around the load. This typically results in an unequal tension being drawn in the strap. The tension is greatest in the (straight) section of strap from the tensioning device to the first corner of the load. The tension is next highest in the straight section of strap from the first corner to the next corner. In turn, the tension is reduced in each section of strap around the load until the leading end of the strap is reached.
To accommodate the unequal tensioning in the strap, one arrangement uses a strapping head (with tensioning device) that is allowed to “float” in the direction of strap feed (with a spring return) or is pushed mechanically (such as by a cylinder) in the float direction to try to equalize the tension in the strap all around the load.
Neither of these equalizing arrangements have been found to be fully effective because the external force is applied to the strap in only one direction. Moreover, both of these arrangements require additional parts and/or assemblies to float and return the sealing head from the float position.
Accordingly, there is a need for a strap tension equalizing assembly that equalizes the tension in the strap on both sides of and in both directions from the strapping head. Desirably, such a device applies an approximately equal force in both directions on the strap. More desirably, such a device applies a force in both directions without the need for cylinders or springs to balance the device.